Breast cancer is a disease with high incidence in the U.S. and elsewhere, and population-level methods of fighting this disease are aimed primarily on screening, using mammography for early detection. The median size of breast cancer found using mammography is approximately 11 mm. Based on extensive preliminary studies involving computer simulations, physical measurements, and cadaver breast imaging, we have found that breast CT may be able to routinely detect much smaller breast tumors, in the 3 to 5 mm range. Importantly, the radiation dose of breast CT performed at 80 kVp was found in detailed studies to be comparable to that of mammography. It is not possible to image the breast alone on a live woman using a clinical CT scanner. Therefore, in this Bioengineering Research Partnership proposal, we have teamed with scientists from around the country to design, build, and test a CT scanner designed to image the breast. A team comprised of medical physicists, physicians, mechanical and electrical engineers, and breast cancer advocates will collaborate on the design of the breast CT scanner. Cone beam flat panel technology will be used to produce a scanner capable of 10 second breast scanning, and the scanner development will also include a breast immobilization system (acrylic cylinders), a breast CT table, a fast reconstruction computer, and a computer workstation customized for efficient viewing breast CT images. The scanner will be built, tested, and optimized at UC Davis over a period of 3 years involving 9 specific aims. After the breast CT scanner is tested in a brief phase I trial (2 specific aims), it will be moved to the breast imaging clinic for a phase II trial where approximately 120 women will be imaged (4 specific aims). This phase II trial will evaluate the efficacy of breast CT for the early detection of breast cancer in a group of women likely to have breast cancer (BIRADS 4 & 5). Additionally, the breast image data will be studied for its utility in automating the analysis of the normal breast architecture, and for computerized cancer detection. In year 5 of the proposed research, two specific aims utilize the breast CT data and corresponding mammography images (on -240 breasts) to evaluate the ideal observer performance and human (mammographer) detection performance attributes of the breast CT scanner. At the end of the proposed research involving 17 specific aims, the potential of breast CT will have been evaluated both qualitatively and quantitatively. A tested, high quality prototype breast CT scanner would be ready to be enlisted in a phase III trial (beyond the scope of this proposed research), if further testing is warranted. Performance data acquired in the present study would allow the proper design (power, etc.) of a phase III trial. If breast CT lives up to its enormous potential based on initial imaging, breast cancer would be detectable far before metastases occurs - for example, a 3 mm tumor contains only 2 percent of the cell count of an 11 mm lesion, and a 5 mm lesion contains only 9 percent of the cell count. Based on a 100 day volume doubling time, detection of a 5 mm lesion would lead to 0.93 year earlier detection, and routine detection of 3 mm lesions would result in 1.5 year earlier detection over mammography. Surgical removal of early cancers will effectively result in cure for the majority of women screened using this technology. While breast CT would probably improve cancer detection in all women, some women may have risk factors (dense breasts, genetic markers, etc.) that particularly warrant screening using breast CT. The Phase II trial will shed more light on this issue.